Providing an accurate picture of weather conditions in a pilot's area of interest has been proposed to be a highly useful application for ground-to-air data links. A problem with using data links to transmit such weather pictures is the large number of bits required to exactly specify a weather image. In "Weather Map Compression for Ground-to-Air Data Links", Proceedings of the Aeronautical Telecommications Symposium on Data Line Integration, May 15-17, 1990, sponsored by the FAA and Aeronautical Radio Incorporated in Annapolis, Md., herein incorporated by reference, Dr. Jeffrey L. Gertz presents two compression approaches: A Huffman-Type run length and coding scheme, and an approach called the "Polygon Ellipse" (PE) method.
The Polygon-Ellipse encoding method represents weather regions as ellipses, polygons, and exact patterns. The actual ellipse and polygon parameters are encoded and transmitted. The decoder then redraws the shapes from their encoded parameter values and fills in the included weather pixels. The PE method achieves high levels of compression because most weather regions are roughly elliptical or polygonal, and it generally takes fewer bits to specify the shape parameters of the region than to specify each of the pixels making up the region.
For a pilot's purposes, a weather map made up of an array of between 64.times.64 (4096) and 256.times.256 (65,536), pixels is believed to provide sufficient information. Also, such an array is about all that is viewable on a typical aircraft cockpit display. Each pixel can indicate one of the seven National Weather Service weather levels and, thus, may be specified with three bits. A weather map image may therefore consist of between 12,288 and 196,608 bits of information, not counting any data link overhead. The Mode S data link protocols, for example, can only guarantee the transmission of one or two "Extended Length Message" (ELM-1280 bits) per aircraft, per scan. Other data links may be similarly limited. To transmit weather maps using Mode S protocol, for example, could require between a ten-fold and hundred-fold compression.
The present TDWR (Terminal Doppler Weather Radar), ASR-9 (Airport Surveillance Radar), and NEXRAD (NEXt generation weather RADar) radars are designed to provide hazardous weather information to controllers located at the tower or at an enroute center.